1. Technical Field
The present invention relates to an iron based alloy material for a thixocasting process and to a method for casting the material.
2. Related Art
Thixocasting processes are methods in which a pressure load is applied to a half-melted billet in a solid-liquid coexisting state to perform injection molding into a die. This method enables the formation of parts with thinner walls and more complicated shapes in comparison with conventional forming methods. In this method, production costs can be reduced due to reduction of machined portions, and thermal load to the die is extremely reduced since the casting can be performed at a lower melting temperature than that in an ordinary diecasting process. Therefore, it has been known that thixocasting processes are promising as method for diecasting materials such as cast iron. However, the liquid phase of cast iron produced by thixocasting processes forms a quenched matrix with low toughness. In iron molding using a high-temperature billet, solidification contraction is greatly when cooling in the die, and cracks are easily formed in the quenched matrix.
In Japanese Patent Unexamined (KOKAI) Publication No. 239513/97, there is proposed a method in which formation of cracks can be suppressed by preventing the formation of chill matrix by using a carbon die. However, the carbon die has insufficient strength and the service life thereof is short, and the production efficiency is reduced due to frequent maintenance of the die. In Japanese Patent Unexamined (KOKAI) Publication No. 123242/01 and Japanese Patent Unexamined (KOKAI) Publication No. 144304/00, there is proposed a method in which formation of cracks can be inhibited by strengthening the quenched matrix by adding chromium or by mixing the quenched matrix with a high toughness phase by increasing the content of manganese. However, the die is worn by friction with a product having a hard quenched matrix during solidification contraction thereof since the solidification contraction in the die still occurs to a great extent. When the wear in the die is promoted, the precision in size of a product at the worn portion is degraded and service life of the die is shortened. When a washing provided on the inner surface of the die is partially stripped by friction with the product, the heat conductivity between the die and the product varies at each portion. As a result, the solidification rate varies at each portion, and this results in casting defects such as size variations and cracks.